1. Field of the Invention
The present invention relates to a process for producing a shaped product. More particularly, it relates to a process for producing a shaped product, which is useful for the production of molded products, for example, three-dimensional shaped products having specific shapes including ocular lenses such as contact lenses or intraocular lenses, camera lenses, pickup lenses for compact discs (CDs) or lenses for spectacles, or for the production of compound products such as bone cement for substitution of artificial hip joints.
2. Discussion of the Background
Heretofore, ocular lenses such as contact lenses or intraocular lenses, have been produced by a so-called mold polymerization method wherein a monomer is charged into a mold and polymerized in the mold by using azobisisobutyronitrile or a peroxide as a polymerization initiator.
However, when such a mold polymerization method is employed, there is a drawback that polymerization shrinkage occurs as the polymerization of the monomer progresses in the mold, whereby it is difficult to prepare a molded product corresponding precisely to the shape of the mold.
Further, a technique so-called "Stereo Lightgraphy", has recently been employed in which a polymer is laminated by photopolymerization to obtain a three-dimensional shaped product. In such a method, it is common in many cases to employ an epoxy resin which is less susceptible to polymerization shrinkage. However, if a resin susceptible to polymerization shrinkage, such as an acryl monomer, is used, dimensional precision of the shaped product tends to be low, and it tends to be difficult to obtain a shaped product having a desired shape. Thus, this technique has a problem that the types of useful resins are substantially limited.
As is apparent from the above examples, it is important to reduce polymerization shrinkage in many fields where polymerization curing of a monomer is utilized.
It is usually considered that polymerization shrinkage occurs at the time of polymerization of a monomer, because the van der Waals distance among the monomer molecules changes to a covalent bond distance by the polymerization. Accordingly, the smaller the amount of the monomer involved in the polymerization, the less the polymerization shrinkage. In a case where polymerization is resumed from a partially polymerized state, the degree of polymerization shrinkage decreases to a level corresponding to the amount of the remaining monomer. For example, in radical bulk polymerization of methyl methacrylate, it is known that when polymerization is carried out from the monomer, the polymerization shrinkage will be about 21%. In a case where a mixture partially polymerized to a level of 50%, is further polymerized, the polymerization shrinkage will be reduced to a level of 10%.
Paying an attention to such a point, it has been proposed to fill a partially polymerized material in a predetermined mold or cavity and to polymerize it again to reduce the polymerization shrinkage.
In such a proposal, it has been common to use, as a polymerization initiator, an azo compound such as azobisisobutyronitrile, a thermal polymerization initiator such as a peroxide compound such as benzoyl peroxide, or a photopolymerization initiator such as benzophenone, benzil or a benzoin compound. In the polymerization with these polymerization initiators, once active free radicals are dissociated from the polymerization initiator, they immediately attach to the monomer, and the polymer chains will continue to grow, unless deactivation due to a chain transfer or termination reaction takes place during the polymerization. Accordingly, once polymerization is initiated, it is not possible to terminate the polymerization unless the growing species are deactivated. Once growing species are deactivated, the polymerization ability will not be regained. Such a characteristic makes it difficult to obtain a partially polymerized mixture in a desired state quantitatively. For example, it is difficult to completely terminate polymerization even if the material wherein the polymerization is once initiated, is transferred to a low temperature environment to reduce the apparent reaction rate, whereby there will be a problem that polymerization gradually proceeds during the storage, and the material is thereby cured and becomes useless. Further, from the viewpoint of resuming polymerization from a partially polymerized state, it is impossible to resume polymerization of a partially polymerized mixture which has once been deactivated, from such a deactivated state, and an additional mixing operation will be required to incorporate a fresh polymerization initiator, and such a method can hardly be useful for a common user who has no equipment required for such an operation.